NaSICON materials were first reported by Hong[1] in 1976. The Hong formulation is a series of solid solutions between the end members NaZr2(PO4)3 (also known as NZP) and Na4Zr2(SiO4)3, expressed by the general formula:Na1+xZr2SixP3−xO12   (1)
Where x varies from 0-3 (0≦x≦3). The variable ‘x’ is the amount of Si substituted for P into 1 mole of NZP. Because silicon has a valence of +4 and phosphorus has a valence of +5, an additional mole of Na+ must be added for each mole of P5+ substituted to maintain charge neutrality. While the Si4+ additions are known to directly substitute into the lattice sites formerly occupied by the P5+ ions, the additional Na+ ions are known to occupy one of two or three equivalent lattice sites not fully occupied by sodium ions in any of the NaSICON compositions for which x<3.
Other exemplary NaSICON-type materials are described by J. B. Goodenough et al., in “Fast Na+-ion transport skeleton structures”, Materials Research Bulletin, Vol. 11, pp. 203-220, 1976; J. J. Bentzen et al., in “The preparation and characterization of dense, highly conductive Na5GdSi4O12 nasicon (NGS)”, Materials Research Bulletin, Vol. 15, pp. 1737-1745, 1980; C. Delmas et al., in “Crystal chemistry of the Na1+xZr2−xLx(PO4)3 (L=Cr, In, Yb) solid solutions”, Materials Research Bulletin, Vol. 16, pp. 285-290, 1981; V. von Alpen et a., in “Compositional dependence of the electrochemical and structural parameters in the NASICON system (Na1+xSixZr2P3−xO12)”, Solid State Ionics, Vol. 3/4, pp. 215-218, 1981; S. Fujitsu et al., in “Conduction paths in sintered ionic conductive material Na1+xYxZr2−x(PO4)3”, Materials Research Bulletin, Vol. 16, pp. 1299-1309, 1981; Y. Saito et a., in “Ionic conductivity of NASICON-type conductors Na1.5M0.5Zr1.5(PO4)3 (M: Al3+, Ga3+, Cr3+, Sc3+, Fe3+, In3+, Yb3+, Y3+)”, Solid State Ionics, Vol. 58, pp. 327-331, 1992; J. Alamo in “Chemistry and properties of solids with the [NZP] skeleton”, Solid State Ionics, Vol. 63-65, pp. 547-561, 1993; K. Shimazu in “Electrical conductivity and Ti4+ ion substitution range in NASICON system”, Solid State Ionics, Vol. 79, pp. 106-110, 1995; Y. Miyajima in “Ionic conductivity of NASICON-type Na1+xMxZr2−xP3O12 (M: Yb, Er, Dy)”, Solid State Ionics, Vol. 84, pp. 61-64, 1996.
Numerous variations in the basic Hong formulation have already been reported in the literature. These include ionic substitutions of all four cations (Na, Zr, P, S), some of which include the addition or removal of the Na. The original paper by Hong listed substitutions of Li+, Ag+ and K+ for the Na+ in the crystal structure. Other substitutions have included full or partial replacements of Zr with Ti, In[2, 3] and several other transition metal[4, 5] and rare earth elements. In all cases, for elements with lower valence than +4, additional Na+ is added to the formulation to maintain stoichiometry of the original NaSICON material. Technical papers have claimed this 1) reduces the sintering temperature of the NaSICON; 2) reduces the free ZrO2 in the sintered microstructure; 3) increases the stability of the NaSICON (particularly with water); or 4) increases the conductivity of the material. In any case, the more general formula for NaSICON ceramics can be expressed as containing materials can be expressed as Me1+x+yMIIIyMIV2−ySixP3−xO12, where Me is the alkali or monovalent metal ion, MIII is a +3 cation and MIV is a +4 cation. All materials reported in the literature have the same crystal structure and same basic microstructural characteristics and similar baseline Hong formulations, usually with only minor changes in lattice parameters and ionic conductivities of the ceramic.
Other variations in the formulation of NaSICON reported in the literature have included the making of the ceramic that is Zr-deficient, and in some cases adjusted to be Na-rich to maintain stoichiometry. Papers[11, 12] report the substitution of Na+ into the Zr4+ lattice sites in these compositions. Others report compositions deficient in Zr that are non-stoichiometric, with resulting vacancies in the O2− anion sub-lattice. Most notably is the compositions first reported by von Alpen[13] of Na1+xZr2−x/3SixP3−xO12−2x/3.
Several compositions of MeSICON family of materials are disclosed herein. These materials include NaSICON type materials where the metal ion is sodium and several compositions of alkali-ion conducting ceramic materials based upon the disclosed NaSICON materials. These compositions are fundamentally based on the Hong formulation, first reported on in 1976[1], but vary from Hong's chemistries in one important respect—they are made to be metal ion deficient from the baseline formulation Me1+x+y−zMIIIyMIV2−ySixP3−xO12−z/2. This distinction appears to be unique to all other literature reported on the subject of NaSICON type materials.